Process of improving corrosion resistance of colored oxide coatings on titanium-rich zinc alloys and resulting product



United States Patent US. Cl. 148-6.2 11 Claims ABSTRACT OF THE DISCLOSURE A process for improving the corrosion resistance of a coloured oxide coating formed on a titanium-rich zinc base alloy surface comprising immersing the oxide coated alloy surface in an aqueous salt solution of alkali and dichromate ions, and the product produced thereby.

BACKGROUND OF THE INVENTION This invention relates to the treatment of coloured coatings formed on zinc-rich alloy surfaces and, more particularly, is directed to the improvement of corrosion resistance of coloured oxide coatings formed on titaniumrich zinc base alloy surfaces.

The production of coloured zinc coatings is disclosed in co-pending United States patent application Ser. No. 574,684, filed Aug. 24, 1966 and in Canadian patent application Ser. No. 965,078, filed July 11, 1966. The process disclosed and claimed in those applications involves the provision of a thin oxide film having light interference colour characteristics on a zinc-rich alloy coating. It was discovered, inter alia, that the presence of an oxygen-avid element selected from the group consisting of titanium, manganese and vanadium alloyed with the zinc in predetermined amounts, permitted the production of the oxide films.

Although aesthetic and reproducible colour effects could be obtained by the process of the invention of application Ser. No. 574,684, and corrosion resistance of the coloured surfaces was improved over normal bright galvanized surfaces, the coloured surfaces were vulnerable to marking or staining during handling by fingerprinting, during damp storage indoors due to discolouration, and during outdoor exposure due to colour deterioration.

It is known that chromate coatings provide improved corrosion resistance to galvanized surfaces. Chromate coatings are formed by conversion of the metal surface by chemical reaction with hexavalent and/or trivalent chromium ions to yield an amorphous chromium oxide gel, in which Zn'CrO may be formed or absorbed. However, since the colour effects noted above are a function of oxide film thickness and refractive index, it was expected that chromate conversion coatings would destroy or alter the attractive colour and metallic sheen provided by the oxide film. This proved to be the case with manganese-rich and vanadium-rich zinc base alloys.

SUMMARY OF THE INVENTION I have discovered that the treatment of coloured coatings formed on titanium-rich zinc base alloy surfaces by 3,546,028 Patented Dec. 8, 1970 chromating said surfaces in an aqueous solution of hexavalent chromium ions provided by an alkali dichromate such as sodium dichromate, potassium dichromate or ammonium dichromate at a pH in the range of from about 0 to 5 surprisingly results in the coloured coatings having markedly improved corrosion resistance with no visible colour change.

It is a principal object of the present invention, therefore, to provide a treatment for improving the corrosion resistance of coloured coatings formed on titanium-rich zinc base alloy surfaces.

It is another object of the present invention to provide a simple and inexpensive treatment for improving corrosion resistance of coloured galvanized coatings which can be readily integrated with galvanizing line operations.

And another object of the invention is the provision of a protective corrosion resistant coating which will not visibly alter light interference effects of oxide films formed on titanium-rich zinc base alloy surfaces and thus not alter surface colours.

These and other objects of the invention and the manner in which they can be attained will become apparent from the following detailed description of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS In accordance with the process of my invention, a coloured surface of titanium-rich zinc base alloy having an oxide coating formed thereon, such as for example, an alloy of zinc with at least about 0.001% by weight titanium treated according to the process of the above co-pending applications, is immersed in an aqueous solution containing hexavalent chromium ions, the said solution being maintained at a pH in the range of from about 0 to about 5. After immersion of the surface, the excess chromate solution is removed by washing the treated surface with Water. The surface is then dried.

The form in which the hexavalent chromium ions are introduced into the solution does not appear critical. Satisfactory solutions can be obtained commercially or can be prepared from, for example, reagent grade sodium dichromate (Na Cr O .2H O) and de-ionized water. In carrying out the process of the invention using sodium dichromate, said hexavalent chromium ions should be present in amount stoichiometrically equivalent to from about 0.5 to about 1000 grams of said reagent grade Na Cr O .2H O per litre of solution, i.e., 0.5-1000 g.p.l.

The pH of the aqueous solution containing hexavalent chromium ions preferably should be maintained in the range of about 0.5 to 2 for immersion coating without applied polarity; in the range of 3-5, preferably about 4, for anodic polarity of the coating; and in the range of 1.5-2.5, preferably about 2, for cathodic polarity of the coating. The pH can be readily controlled by the addition of a mineral acid, such as sulphuric acid, or sodium hydroxide, as will be evident to the artisan.

The solution temperature can be maintained at ambient temperature (about 20 C.) or at temperatures up to the boiling point of the solution, preferably in the range of about 65-85 C., for an increased reaction rate. The upper range of solution temperature can be readily achieved and maintained by the incorporation of the chromating treatment in a continuous galvanizing line whereby partially cooled galvanized sheet or strip having the coloured coating formed thereon is introduced into 3 the chromating bath at a temperature up to about 100 C.

The time of immersion of the coloured surface in the bath of aqueous solution containing hexavalent chromium ions can be varied over a wide range, it being important that suflicient minimum retention time in the solution be provided to ensure an adequate chromate coating for corrosion resistance. A retention time of from about 3 seconds up to about 60 seconds has been found satisfactory for production of an effective coating, a retention time of about seconds permitting incorporation of the process of the invention in a continuous line treatment. Optimum retention time will vary as to concentration of the dichromate salt, pH of the aqueous solution, and the solution temperature, as will become evident from the following examples.

The nature of the reaction for the formation of the coating film is not known precisely, but it is believed that the aqueous solution containing hexavalent chromium ions infiltrates the oxide coating, which is of a porous and permeable nature, and reacts chemically with the underlying zinc-titanium alloy to yield a chromium oxide gel while not affecting the original oxide film. Upon drying, the chromium oxide gel does not visibly affect the colour characteristics of the coating and thus does not alter the effective thickness or the refractive index of the oxide film.

The practical limits for pH, immersion time and solution temperature were determined in part by colour stripping of the coating. Table 1 illustrates the period of time to first signs of colour stripping of coloured titanium-rich zinc base alloy surfaces (2nd order pale blue) formed on 3 inch by 4 inch, gauge galvanized steel panels immersed in an aqueous sodium dichromate solution having a hexavalent chromium ion concentration stoiehiometrically equivalent to 50 g.p.l. of Na Cr- O -2H O. Colour stripping is considered to occur at the first visible indication of solution attack on the substrate to yield an irregular hairline crack pattern.

Table 2 below illustrates the time to first signs of colour stripping of panels having coloured titanium-rich zinc base alloy surfaces of the type discussed for Table 1 immersed in an aqueous solution of sodium dichromate having a hexavalent chromium ion concentration stoichiometrically equivalent to 200 g.p.l. of Na Cr O -2H O.

TABLE 2.'IIME TO COLOUR STRIIPING Time (mins.) pH Time (mins.)

Table 2 also shows that the time to colour stripping is significantly shortened at increased temperatures and reduced pl-Is; indicating an accelerated rate of reaction. It will be noted from a comparison of Tables 1 and 2 that an increase in hexavalent chromium ion concentration results in a further shortened time to colour stripping.

The corrosion resistance of chromated oxide films produced according to the process of the present invention was determined for ranges of pH, immersion times and sodium dichromate concentrations. Table 3 indicates hours of resistance to a neutral 5 percent sodium chloride salt spray according to ASTM Bl176l, i.e., to first indications of white corrosion product, for 3 inch by 4 inch, 20 gauge galvanized steel panels having (i) bright galvanized surface only, (ii) unchromated coloured titanium-rich zinc base alloy surface (2nd order pale blue) and (iii) coloured titanium-rich zinc base alloy surface (2nd order pale blue) chromated by immersion in an aqueous solution containing 200 g.p.l. of Na Cr O -2H O at a bath temperature of 20 C. The time was varied for each pH to avoid colour stripping.

Table 4 indicates salt spray resistance of similar panels tested under the conditions discussed for Table 3 but with an increase in bath temperature to 65 C.

TAB LE 3 Corrosion resistance vs. chromating conditions 20 C. and 200 g.p.l. Ntl2Cl'207-2Hz0 Surface pH Time, Appearance before corrosion Salt mins. testing spray, hrs.

Galvanized Bright 1 Unchroinated oxide film Original colou 4 5 No change in original colour. 20 3.75 do 45 1.17 do 64 0.42 Slight fading 190 TABLE 4 Corrosion resistance vs. chromating conditions 65 C. and 200 g.p.l. NflzGHOrQI-LO Surface pH Time, Appearance before corrosion Salt mins. testing spray, hrs.

Galvanized Bright 1 Unehroniated oxide film Original colon 4 Chromated oxide film... .8 No change in ori .3 2.25 do .8 0.75 ...do 150 Do 1. 0 0.167 Slight fading 175 TABLE 1.TIME TO COLOUR STRIPIING Time (mins.) pH Time (mins.)

It will be noted, from a comparison of Tables 3 and 4, that corrosion resistance is increased by chromating at the higher temperature. Thus, in the solutions of pH 2.3 and 1.0, despite a markedly shorter immersion time at the higher temperature, the corrosion resistance of the oxide surfaces chromated at 65 C. Was about the same as that of those chromated at 20 C., while in the solutions of pH 3.8 and 1.8, in which the immersion times at each temperature differed relatively little, chromating at 65 C. gave greatly increased corrosion resistance. The shorter immersion times at the higher temperature in the solutions of lower pH were necessary to avoid colour stripping, as previously discussed with reference to Tables 1 and 2. I

Tables 5 and 6 indicate the results of tests identical to those carried out for Tables 3 and 4 above on similar panels but employing an aqueous solution of sodium 3,546,028 e dichrornate having 50 g.p.l. of Na Cr O -2H O. Table Three test panels of 20 gauge galvanized steel coated with 5 indicates the COI'IOSIOH resistance provided by a bath titanium alloy (second order pale blue) were immersed malntamed at 20 C. and Table 6 indicates the corrosion for 2 minutes in a 4% solution of sodium hydroxide,

resistance provided by a bath maintained at 65 C. then washed in water. Each panel was dipped for TABLE 5 Corrosion resistance vs. chromating conditions C. and 50 g.p.l. Na Cr O -2H O Time, Appearance before corrosion Salt Surface pH mins. testing spray, hrs.

Galvanized Bright 1 Unchromated oxide film Original colon 4 Chromated oxide fi1m 3. 9 15 No change in original colou Do 2.9 5 do Do 1. 75 3. 5 Slight fading 200 Do 1. 00 0. 75 No change in original colour 175 TAB LE 6 Corrosion resistance vs. chromating conditions 65 C. and 500 g.p.l. N82Cr2O7-2Hz0 Time, Appearance before corrosion Salt Surface pH mins. testing spray, hrs.

Galvanized Bright 1 Unehromated oxide film Original colour. 4 Chromated oxide film 9 5 No change in ori 40 Do .9 5 50 Do. .75 64 Do... 00 r 64 Corrosion resistance of chromate films provided at seconds in a separate dichrornate solution maintained at pHs of 1.75 and 1.0 proved inferior at the higher bath room temperature which had been prepared as follows: tem erature. However, this may be due to the considerably reduced immersion times at the higher bath temp- SOLUTION 1 erature necessary to avoid colour stripping. 30 To a 574 solution of NaZCQOTZHZO having Table 7 shows h results of Fests in which f Pi- 2.00 g.p.l. Cr and a pH of 4.08 was added 0.30 ml./l. tive was to determine the chromium concentration glVlIlg H2301 (3G. The final PH was 2'0 the maximum corrosion resistance. Test panels of 20 gauge galvanized steel coated with titanium allow (second 3r SOLUTION 2 order ale blue were dipped in a 4% solution of sodium 0 hydror r ide for tzvo minutes, washed in water, then dipped To a solutlon of z 'l havmg -pin aqueous solutions of sodium dichrornate for 15 Cr and a PH 0f 405 was added 035 ml/ 2 4 seconds. Each test panel was dipped in a solution of a The final PH was different concentration, the concentrations being set out 40 SOLUTION 3 in the table. The solutions were maintained at 20 C. The panels were then washed twice in water and dried To a 5.53 g.p.l. solution of (NH Cr 0 having 2.27 for 24 hours at room temperature. All specimens were g.p.l. Cr and a pH of 4.00 was added 0.25 ml./l. H 80 tested for corrosion resistance in a neutral 5% sodium (8.6. 1.84). The final pH was 2.0. chloride salt spray according to ASTM standard The panels were washed in Water twice and dried in B-l17-61. air at room temperature for 24 hours. None of the Formation of white corrosion product Chromate concentration First appearance At removal from test G. .l.N CrO G. .1. Coverage, Coverage,

I: M 2 7 p r Time, hrs. percent Time, hrs. percent Estimated.

It will be observed that the maximum corrosion rechromating solutions affected the original colour of the sistance occurring in the range of concentration of from material. Table 8 indicates hours of reslstance to a 0.06 to 20 grams per litre chromium is at about 6.00 neutral 5% sodium chlorlde salt spray according to g.p.l. chromium. ASTM standard B-1l7-61.

TABLE 8.EFFECT OF CATION ON RESISTANCE TO SALT SPRAY [20 C., immersion time 15 sec.]

Formation of white corrosion product First appearance At removal from test Time, Coverage, Time, Coverage, Salt GmJl. Gm./l. Cr hrs. percent hrs. percent Solution 1- NB2CI'207'ZHZO 5. 74 2. 00 55 10 79 50 SolutionZ K2Cl'207 6.50 2. 29 79 2 86 30 Solution 3 (NHDZCYZOI' 5. 53 2. 27 55 5 79 50 Table 8 provides a comparison of the corrosion re- Table 8 indicates that the protection aliorded by the sistance of separate panels chromated in aqueous solpotassium salt is better than the protection afforded by tions of potassium-, sodium and ammonium dichrornate. the others, namely 79 hours to first sign of white corrosion product for the potassium salt as compared to 55 hours for the sodium and ammonium salts.

Panels treated as discussed above with reference to Tables 1-6 were subjected to a degreasing pre-treatment by immersion in an organic solvent, such as acetone, then in an aqueous solution containing 4% sodium hydroxide followed by a water wash, to remove dirt particles and hydrophobic films produced during exposure to the atmosphere. A degreasing pre-treatment normally would not be required if the process of the present invention was integrated with a galvanizing line.

Anodic and cathodic treatment of 20 gauge, galvanized steel panels having coloured zinc-rich titanium alloy surfaces formed thereon (2nd order pale blue), were conducted at several current densities within the range of from about 1 to about 20 amperes per square foot in an aqueous solution containing 200 g.p.l. of Na Cr O .2H O maintained at a temperature of 24 C. at a pH of each of 1.5, 3.5 and 5.0. The tests indicated that although the anodic treatment was superior to the cathodic treatment for colour retention and corrosion resistance, both treatments were effective in improving corrosion resistance.

The present invention provides a number of important advantages. The attractive colour provided on zinc-rich titanium alloy surfaces by the process disclosed in copending US. application Ser. No. 574,684 and Canadian application Ser. No. 965,078 can be protected indefinitely for indoor use and the corrosion resistance in exterior locations can be substantially enhanced. The application of a protective chromate film can be readily accomplished as a process step integrated with a continuous galvanizing line, or provided in batch operations after a simple cleaning operation to ensure that the surface to be treated is clean and receptive to the chromating solution.

It will be understood, of course, that modifications can be made in the preferred embodiments of the invention described and illustrated herein Without departing from the scope and purview of the appended claims.

What I claim as new and desire to protect by Letters Patent of the United States is:

1. A process for improving the corrosion resistance of a coloured oxide coating formed on a zinc base alloy containing at least 0.001% titanium surface which comprises immersing said oxide coated alloy surface in an aqueous salt solution containing alkali and dichromate ions in amount stoichiometrically equivalent to from about 0.5 gram per litre to about 1000 grams per litre sodium dichromate, said aqueous solution having a pH in the range of from about '0 to 5. p

2. In a process as claimed in claim 1, said aqueous solution containing alkali and dichromate ions in amount stoichiometrically equivalent to from about 0.5 gram per litre to about 200 grams per litre sodium dichromate.

3. In a process as claimed in claim 2, said alkali ions being selected from the group consisting of sodium, potassium and ammonium ions.

4. In a process as claimed in claim 3, subjecting said oxide coated alloy surface to a cleaning pre-treatment which comprises degreasing said oxide coated alloy surface by immersion in an organic solvent and in an aqueous alkaline cleaning solution, and water washing said oxide coated alloy surface.

5. In a process as claimed in claim 3, maintaining the pH of the aqueous salt solution in the range of from about 0.5 to about 2.0.

6. In a process as claimed in claim 3, immersing said oxide coated alloy surface in the aqueous salt solution for at least about 3 seconds.

7. In a process as claimed in claim 3, immersing said oxide coated alloy surface in the aqueous salt solution for a period of time between 3 seconds and 60 seconds.

8. In a process as claimed in claim 7, maintaining said aqueous salt solution at about ambient temperature.

9. In a process as claimed in claim 7, maintaining said aqueous salt solution at a temperature within the range of from about ambient temperature up to about 100 C.

10. In a process as claimed in claim 8, subjecting said oxide coated alloy surface to a cleaning pre-treatment which comprises degreasing said oxide coated alloy surface by immersion in an organic solvent and in an aqueous alkaline cleaning solution containing at least 4% sodium hydroxide and water washing said oxide coated alloy surface.

11. An article having a coloured oxide surface of zinc base alloy containing at least 0.001% titanium provided with a corrosion resistant film prepared by contacting said surface with an aqueous solution of an alkali dichromate selected from the group consisting of sodium dichromate, potassium dichromate and ammonium dichromate.

References Cited UNITED STATES PATENTS 1,978,265 10/ 1934 Ivins et a1. 1486.3X 1,985,784 12/1934 Jordan 1486.3UX 2,018,388 10/1935 Tosterud 148-6.2UX 2,203,670 6/1940 Buzzard 148-6.2X 2,210,850 8/1940 Curtin 1486.2 2,236,549 4/1941 Darsey et a1. 148-6.2X 2,295,063 9/1942 Tuttle 1486.2X 2,393,663 1/1946 Thomas et al 1486.2 3,056,694 10/1962 Mehler et al. 148-6.3UX

ALFRED L. LEAVI'IT, Primary Examiner J. R. BATTEN, JR., Assistant Examiner US Cl. X.R. 1486.3 

